In recent years, in the field of disk storage apparatuses such as hard disk drives (hereinafter, simply as disk drives), higher recording densities of a magnetic disk as a storage medium have been promoted. In a disk drive, generally, the sector servo system is adopted and a large number of circular cylinders (i.e. tracks) including servo sectors are arranged concentrically on the magnetic disk. The servo sector includes a servo area and a user data area subsequent to the servo area. A servo pattern to detect position of a magnetic head over the magnetic disk is recorded in the servo area. The servo pattern includes a preamble pattern, address data containing a cylinder number and a servo sector number, and a burst pattern.
There is known a staggered servo system which is one of variations of the sector servo system. In a magnetic disk apparatus in the staggered servo system, both sides of the magnetic disk are used as a recording surface respectively and the magnetic head is provided over both sides of the magnetic disk respectively. There is provided a positional difference between the servo sectors on the both sides of the magnetic disk along the circumferential direction of the disk and the servo areas are also recorded on the both side so as to have a positional difference in the circumferential direction depending on the positions of the sectors.
In such a magnetic disk apparatus, one of the magnetic heads is activated and the servo pattern is sampled at servo sampling intervals by the magnetic head from the corresponding recording surface of the magnetic disk. The magnetic head is positioned to the target track based on the sampled servo pattern. More specifically, a demodulated position and an estimated position of the magnetic head are determined from address data contained in the servo pattern sampled at servo sampling intervals and a demodulated velocity and servo control to the target track position of the magnetic head is exercised. Then, if off-tracked writing error in which the magnetic head is drifted over a permissible position error (threshold dp) is detected in the servo control while writing data, the magnetic head is disabled to write data.
In the servo control involving off-tracking detection of the magnetic head as described above, even if a rapid velocity change of the magnetic head is caused and off-tracked writing error is occurred after the magnetic head passes particular servo sample, the detection of the off-tracking is delayed to the next servo sample. Accompanying such a delay of off-tracking detection, data writing may continue even if the magnetic head is over an adjacent track due to off-tracking, resulting in deletion of data of the adjacent track. In addition, if the servo sampling interval is set small to minimize the delay of off-tracking detection, the user data area is decreased.